Don't raise error when empty community emoji setting submitted (#15723)

* Don't raise NoMethodError when validating emoji settings

Cargo culted the unique cross model slug validator setup (which is why
value is called name here in the validatable).

Don't raise an error when validating a nil emoji, and assert nil and
empty string are permitted values.

This expects no non-emoji characters (so an empty string would be
permitted), and nil should be permitted.

I believe this might have been introduced by the string settings
cleaner (exchanging "" for nil in form inputs).

* Update app/validators/emoji_only_validator.rb

guard for value, and then validate value

app/validators/emoji_only_validator.rb

@@ -2,6 +2,7 @@ class EmojiOnlyValidator < ActiveModel::EachValidator
   DEFAULT_MESSAGE = "contains non-emoji characters or invalid emoji".freeze
 
   def validate_each(record, attribute, value)
+    return unless value 
     return if value.gsub(EmojiRegex::RGIEmoji, "").blank?
 
     record.errors.add(attribute, options[:message] || DEFAULT_MESSAGE)

spec/validators/emoji_only_validator_spec.rb

@@ -0,0 +1,60 @@
+require "rails_helper"
+
+RSpec.describe EmojiOnlyValidator do
+  subject(:record) { validatable.new.tap { |m| m.name = name } }
+
+  let(:validatable) do
+    Class.new do
+      def self.name
+        "👍"
+      end
+      include ActiveModel::Validations
+      attr_accessor :name
+
+      def initialize
+        @enforce_validation = true
+      end
+
+      attr_accessor :enforce_validation
+      alias_method :enforce_validation?, :enforce_validation
+
+      validates :name, emoji_only: true, if: :enforce_validation?
+    end
+  end
+
+  context "when if option is false" do
+    let(:name) { "not-an-emoji" }
+
+    before { record.enforce_validation = false }
+
+    it { is_expected.to be_valid }
+  end
+
+  context "when name includes non-emoji" do
+    let(:name) { "not-an-emoji" }
+
+    it { is_expected.not_to be_valid }
+  end
+
+  context "when name is an emoji" do
+    let(:name) { "🍀" }
+
+    it { is_expected.to be_valid }
+  end
+
+  context "when name is nil" do
+    let(:name) { nil }
+
+    it "does not raise no method error" do
+      expect { record.valid? }.not_to raise_error
+    end
+
+    it { is_expected.to be_valid }
+  end
+
+  context "when name is empty" do
+    let(:name) { "" }
+
+    it { is_expected.to be_valid }
+  end
+end